Reverse magnetron having a circular electric mode purifier in the output waveguide



April'29, 1969 D #055 ETAL 3,441,793

I N HAVING A REVERSE MAGNE RCULAR ECTRIC MODE PURIFIER IN THE OUTPUT WA DE Filed July 8, 1966 INVENTORS PETER nu FOSSE GEORGE EGLENFIELD" United States Patent U.S. Cl. 315-3953 3 Claims ABSTRACT OF THE DISCLOSURE A reverse magnetron microwave tube is disclosed. The tube includes a circular electric mode cavity resonator surrounded by an array of vane resonators coupled to the fileds of the circular electric mode resonator via the intermediary of an array of axially directed slots. A cathode emitter surrounds the vane resonators to define a magnetron interaction region therebetween. A magnetic circuit which includes a magnetic pole structure provides an axially directed magnetic field in the interaction region. The pole structure includes an axially directed bore containing a tubular output waveguide operable in the circular electric mode for coupling output wave energy from the circular electric mode cavity to a suitable load. A microwave window is sealed across the output waveguide internally of the magnetic pole structure. The output waveguide includes an array of slots communicating through the wall of the tubular waveguide to provide coupling between a lossy mode absorbing element and the fields of the waveguide for selectively absorbing wave energy of the non-circular electric mode in the waveguide, thereby purifying the output wave energy to the desired circular electric mode.

The present invention relates in general to reverse magnetron microwave tubes and, more Particularly, to an improved reverse magnetron having a circular electric mode purifier in the output waveguide, whereby pure circular electric mode output is obtained. Tubes employing features of the present invention are especially useful as output tubes for high peak power radars operating in the frequency range of 30 gI-Iz. or higher.

Heretofore, reverse magnetrons have been built. Such tubes are characterized by a centrally disposed circular electric mode cavity resonator coupled to an array of outwardly directed vane resonators which ineract in a magnetron interaction region with a stream of electrons produced by a cathode emitter ring which surrounds the anode array. The output is taken out axially of the tube via a circular electric mode waveguide coupled to the circular electric mode cavity. Such a prior art tube is described in copending U.S. patent application 219,702 filed Aug. 27, 1962 now issued as U.S. Patent 3,255,377 on June 7, 1966 and assigned to the same assignee as the present invention.

It has been discovered that there is a tendency in these tubes for the output wave energy to convert from the desired TE mode to other non-circular electric modes. The precise origin for this conversion process is not fully understood. However, the dominant mode in the output waveguide and circular electric mode cavity is ice not the operating TE mode and it is known that perturbations in the output Waveguide such as those associated with the output microwave window assembly or output coupling device, which couples energy from the circular electric cavity, can produce a conversion of TE mode energy into non-circular modes. Such conversion is troublesome since the utilization devices to which the tube supplies power are designed for the TE mode and substantial amounts of power in other modes can cause arcing and burnout of such devices and unwanted radiation of interfering power.

In the present invention, the output waveguide assembly is provided with an integral mode absorber for absorbing power in such non-circular electric modes to prevent their being transmitted from the tube to the utilization device. In a preferred embodiment, the mode absorber takes the form of an array of transversely directed narrow slots in the tubular output waveguide assembly. The slots open outwardly into a concentrically disposed mode absorber for absorbing the energy from such noncircular electric modes. The transverse slots and mode absorber element do not interfere with the desired TEOII mode as the currents associated with this mode are all parallel to the slots and the slots are relatively narrow. Thus, the output of the tube incorporating the mode absorber is a pure TE mode.

The principal object of the present invention is the provision of an improved reverse magnetron microwave tube having essentially a pure TE mode output.

One feature of the present invention is the provision of a selective mode absorber built as an integral part of the tube in the output waveguide of a reverse magnetron for absorbing the energy in non-circular electric modes of the output signal, whereby the output is purified to the desired TE mode.

Another feature of the present invention is the same as the preceding feature wherein the selective mode a'bsorber is formed by an array of transverse slots cut through the wall of the tubular output waveguide and backed by energy absorbing means for absorbing the energy of the unwanted non-circular electric modes coupled to by the slots.

Another feature of the present invention is the provision of the selective non-circular electric mode absorber in the output waveguide between the microwave window and the tubes output fiange whereby the mode absorber is located externally of the tubes vacuum envelope.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a perspective view of a reverse magnetron employing features of the present invention,

FIG. 2 is an enlarged sectional view of the structure of FIG. 1 taken along line 2-2 in the direction of the arrows, and

FIG. 3 is an enlarged sectional view of the structure of FIG. 2 taken along line 3-3 in the direction of the arrows.

Referring now to FIG. I there is shown an external perspective view of a reverse magnetron tube 1 incorporating features of the present invention. The tube 1 in- I cludes a generally cylindrical central body structure 2, as

of copper, which contains the microwave anode circuit and cathode emitter and is evacuated to a low pressure as of torr. A lead-in high voltage insulator assembly 3 depends from the main body 2 to bring in the cathode voltage, as of -23 kv. A tuner housing assembly 4 is mounted on one axial end of the main body 2 and an output circular electric mode waveguide and output Window assembly 5 is mounted on the other axial end of the main body 2.

Referring now to FIG. 2 the tube will be described in greater detail. A circular electric mode cavity resonator 6 is centrally disposed of the tube main body on its central axis. The cavity resonator 6 as defined by a region bounded on the sides by a cylindrical anode wall 7 and on one end by a conductive end wall 8 and on the other end by a tuning disk 9, forming the other end wall of the cavity 6.

An array of vane resonators 11 project outwardly from the cavity sidewall 7. An array of axially directed coupling slots communicate through the side wall 7 to alternate ones of the vane resonators for locking the 1r mode of the vane resonator system to the TE mode of the cavity resonator 6. A cathode emitter ring structure 12 surrounds the vane resonator array 11 to define an annular crossed field electronic interaction region 13 therebetween. A pair of axially spaced cylindrical magnetic pole pieces 14 and 15 are disposed on opposite sides of the interaction region 13 to provide an axially directed magnetic field therethrough which is orthogonal to the electric field between the cathode ring structure 12 and the vane array 11, operating at anode potential. A pair of C- shaped magnets 16, only partially shown, are coupled to the main body 2 externally thereof for supplying the magnetomotive force to the pole pieces 14 and 15.

The 1r mode magnetron operation of the vane resonator array supplies energy to the TE cavity 6. An output circular electric mode waveguide 17 is axially disposed of the tube 1. A wave permeable vacuum tight window 18 is sealed across the waveguide 17. Output wave energy at the operating frequency of the tube, such as 35 gHz., is coupled out of the cavity 6 around the perimeter of the disk 9 and through the waveguide 17 to a suitable load or utilization device, not shown. A conductive rod 19, mounted on the center line of the cavtiy 6, supports the disk 9 and also serves as the cavity tuner by varying the axial position of the disk 9. The rod 19 is actuated via conventional means and is sealed to the tube body 2 via a gas-tight fiexible bellows 21.

The cathode emitter ring structure 12 is supported from the end wall 22 of the cylindrical tube body structure 2 via a plurality of axially directed insulator assemblies 23 and spring fingers 24.

Referring now to FIGS. 2 and 3 the output selective mode absorber or purifier will now be described in detail. An array of transverse slots 31 are cut through the side wall of the cylindrical output waveguide 17 in the region between the output window 18 and an output waveguide mounting flange 32. The slots 31 are, for example, 0.032" wide and are provided in pairs which enter the guide 17 in the transverse plane, but from opposite sides of the guide 17, such that each slot 31 subtends about 150 of circumferential arc of the guide 17. The pairs of slots 31, as of 7 in number, are axially spaced apart by a metal strip, which, for example, is also 0.032 wide such as to provide equal widths of metal and slot taken in the axial direction of the guide 17. Also axially adjacent pairs of slots 31 are staggered such that they cut into the guide from sides at right angles to each other. In this manner, the axially conducting path left between the ends of the slot pairs 31 is caused to meander around the successive circumferentially overlapping pairs of slots 31, whereby axial currents are heavily coupled to the slot-s 31.

The waveguide 17 is formed by a cylindrical tube, as of brass, which is nickel plated. Typical dimensions of the tube 17 at 35 gHz. are inside diameter 0.594", outside diameter 0.697" and a length of 1.358". The outside diameter of the tubular waveguide 17 is cut down in the region surrounding the slots 31 to provide a wall thickness of 0.015" or about one-half the slot width to further increase coupling of wave energy therethrough. A cylindrical wave energy absorbing member 33, as of carbon impregnated alumina ceramic 0.627" I.D., 0.687 OD. and 0.875" in length, surrounds the slotted region of the output guide for absorbing wave energy coupled thereto via the slots 31. Both the output waveguide 17 and the mode absorber 33 are concentrically disposed of the cylindrical pole piece 14. The energy absorber 33 forms a close mechanical fit over the waveguide 17 with only about 0.001" clearance. The pole piece 14 is formed with about 0.005" cold clearance to the outer diameter of the absorber element 33. Heat generated in the absorber 33 by dissipation of wave energy therein is conducted from the absorber 33 to the pole piece thereby serving as a heat sink for the absorber 33.

In use, non-circular electric mode wave energy in the TE mode output signal, as coupled out of the circular electric cavity 6, is heavily coupled to the lossy mode absorber 33 and dissipated therein. However, the signal wave energy in the desired TE mode is not appreciably coupled to the slots 31 and, thus, passes through the guide unperturbed and emerges from the selective mode absorber region in purified TE form.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrated and not in a limiting sense.

What is claimed is:

1. In a reverse magnetron microwave tube apparatus, means forming a circular electric mode cavity resonator, means forming an array of vane resonators projecting outwardly from said cavity resonator, said cavity resonator having an array of coupling slots communicating between said cavity resonator and said array of vane resonators for coupling together the two resonant systems to form a composite anode microwave circuit, means forming a cathode emitter structure surrounding said vane array in spaced relation to define a crossed field interaction region therebetween for generating microwave energy on said anode circuit, means forming a magnetic circuit for producing an axially directed magnetic field in the crossed field interaction region, said magnetic circuit including a magnetically permeable pole piece structure integral of the tube and having an axially directed bore therein, means forming a circular electric mode output waveguide integral of the tube and coaxially disposed of and Within the bore in said pole piece structure, said waveguide having an output terminal integral of the tube for connecting the tube to a load, such waveguide being axially aligned with said cavity resonator for extracting and propagating microwave energy from said cavity to a load, means forming a microwave permeable gas tight window member sealed across said waveguide intermediate the length of the bore in said pole piece structure, the improvement comprising, means forming an array of slots in said waveguide transversely directed of the longitudinal axis of said waveguide, such slots communicating through the wall of said output Waveguide, said array of slots being disposed intermediate said micro wave window and said output terminal, means forming an energy absorbing member disposed adjacent said slots and between said array of slots in said waveguide and the interior wall of the bore 'in said pole piece structure for selectively absorbing microwave energy of the non-circular electric mode coupled from said output waveguide to said absorbing member via said array of slots, whereby the output microwave energy is purified to the desired TE mode at said output terminal.

2. The apparatus of claim 1 wherein said output Waveguide is tubular, and said energyabsorbing member is tubular and coaxially disposed of said bore in said pole piece and coaxially disposed of said tubular output waveguide.

3. The apparatus of claim 2 wherein the radial wall thickness of the slotted portion of said slotted tubular waveguide is less than the axial width of said slots, whereby increased coupling to said wave absorbing member is obtained.

References Cited UNITED STATES PATENTS 2,901,666 8/1959 Sixsmith 31539.51 X 3,096,462 7/1963 Feinstein 315-39.77 X 3,231,781 1/1966 Liscio 31539.77 X 2,759,15 6 8/ 195 6 Younker.

2,779,006 1/ 1957 Albersheim.

FOREIGN PATENTS 590,302 7/ 1947 Great Britain.

HERMAN KARL 'SAALBACH, Primary Examiner. S. CHATMON, JR., Assistant Examiner.

U.S. Cl. X.R. 

